1. Field of the Invention
This invention relates to a cell assembly for a lithium/metal sulfide battery, and, more particularly, to an improved case for the cell assembly.
2. Description of the Prior Art
Lithium/metal sulfide batteries are becoming increasingly desirable based on their ability to provide high power and energy performance at generally low cost. Such lithium/metal sulfide batteries are typically provided in a cell assembly in which there is at least one positive electrode, one negative electrode and a separator within a can, or case. A typical case for a lithium/metal sulfide cell assembly is shown in FIGS. 1-3, the front, top and side elevational views of the case.
As shown in FIGS. 1-3, the case 10 for a typical lithium/metal sulfide cell assembly has a height 12 and length 14 substantially greater than its width 16. In assembling the cell, the top of the cell is left open and the electrode plates and separator are inserted therein. The electrode plates have a shape similar to the case 10 of the cell assembly. Once the electrode plates are inserted into the case 10, a cover is provided on top of case 10 and is welded thereto.
Because of the shape of case 10, case 10 must be formed of two separate pieces 18 and 20 which are joined at weld line 22. Because of the narrow and deep configuration of case 10, the case cannot be drawn. Rather, case 10 must be manufactured in two halves.
In addition to being difficult to manufacture, traditional prior art cases 10 are also difficult to use during assembly of the battery cell. As previously stated, the electrodes are loaded into case 10 through the top of the case. This requires a compressive load prior to and during the installation of the electrode stack into the case 10. Applying a compressive and sliding load to the electrode plates and separator can result in cracking of the electrodes and separator, as well as cause contamination between the electrodes, thereby potentially developing a short between the electrodes which ultimately causes the cell to fail. Because the electrodes are approximately 100 times taller and 150 times wider than they are thick, the electrodes are difficult to handle without breaking while applying a load to their edge. The electrodes are installed into metal frames at additional cost to protect them from cracking during assembly. Accordingly, there is a need for a low cost cell assembly which provides ease of handling during assembly of the cell.
Side cover battery cases have been used in the past in connection with plastic cases for ambient temperature battery cells. These battery cells are flooded electrolyte systems. In such ambient temperature cells, compression of the electrodes is not as critical as it is in high temperature lithium/metal sulfide battery cells. The need for the compressive load on the electrode stack is not an issue with such ambient temperature battery cells housed in plastic cases.